The enthalpies of dilution of phosphate solutions at 30°C

Abstract

The enthalpies of dilution of aqueous solutions of HCl, H₃PO₄, NaOH, NaH₂PO₄, Na₂HPO₄ and Na₃PO₄ in the molality range 0.1 to 1.0 mole-kg⁻¹ have been determined at 30°C. The relative apparent molal enthalpies φ_L of HCl, NaOH, NaH₂PO₄ and Na₂HPO₄ have been determined with the aid of an extended form of the Debye-Hückel limiting law. The relative apparent molal enthalpies for Na₃PO₄ solutions have been corrected for hydrolysis. A value of ΔH ^o_H =9525±150 cal-mole⁻¹ was determined for the heat of hydrolysis of PO ⁻³₄ . This value gives ΔH ^o₃ =3815±150 cal-mole⁻¹ for the ionization of H₂PO ⁻₄ , which is in good agreement with the value of ΔH ^o₃ =3500±500 cal-mole⁻¹ determined directly by Pitzer at 25°C. The relative apparent molal enthalpies for H₃PO₄ solutions have been corrected for ionization. A value of ΔH ^o₁ =−1900±150 cal-mole⁻¹ was obtained for the heat of ionization of H₃PO₄ to H⁺+H₂PO ⁻₄ . This value is in good agreement with the value of ΔH ^o₁ =−2031 cal-mole⁻¹ at 30°C determined by Harned and Owen from the temperature coefficient of the equilibrium constant and ΔH ^o₁ =−1950±80 cal-mole⁻¹ at 25°C determined from calorimetry by Pitzer.